E.g. in contrast to Dave Reeder
I don't use for the positioning of elements the absolute unit "px".
Instead I use the relative unit "em" as suggested by
Rachel Andrew
in her book
The CSS Anthology:

"The em unit will resize as the text resizes,
which should help us avoid any problems with overlapping text
if the user resizes the font in their browser.
For layouts that are predominantly text-based,
the em is an excellent choice for setting the widths of boxes and margins."